1. Field of the Invention
The present invention generally relates to electronic monitoring systems. More specifically the present invention relates to an improved local monitoring unit for enhancing the tracking and locating capability of a monitoring system. The present invention further relates to a local monitoring unit which comprises an attenuation mechanism in order to achieve a better distance estimation resolution.
2. Discussion of the Related Art
Electronic monitoring systems for remote monitoring and supervising of moving objects, and in particular for monitoring persons, are known in the art. The advantages of employing such a system in a wide range of applications in a variety of fields, including security, law enforcement, medical and more are known. Decreasing restrictions of the monitored person, decreasing in man-power, discreetness, low cost comparing other monitoring systems, easy installation and mobility, multipurpose operation, are only few examples. A typical monitoring system comprises a plurality of portable transmitting devices, known as “tags” (one or more fixed or mobile), local monitoring unit and a central monitoring station. A tag is a device worn by a monitored person, the tag is capable of sending signals form a local monitoring unit. A local monitoring unit is a device designed to receive and send signals to and from a tag or a number of tags and to and from one or more central monitoring stations. The central monitoring station is ordinarily a computer device for collecting and further processing all the information received by the local monitoring unit. Each tag has an identification code. The identification code may be a unique code specific to the subject with whom the tag is associated, such that the subject can be distinguishably identified. Alternatively, the identification code may be a code typical to a certain group. For example, in a hospital, staff may wear a tag having one identification code while patients wear tags having a second identification code. The local monitoring unit includes a radio frequency (RF) or other signal receiving unit tuned to the frequency of the signals transmitted by the transmitting tags. Typically, the local monitoring unit further includes a microprocessor to allow local processing of the data before sending it to the central monitoring station. The communication between local monitoring units and a central monitoring station may be either wireless communication, such as RF, or by conventional communication lines such as telephone lines, cable TV, WAN, LAN and a like.
When used for monitoring persons, the portable tag is usually strapped around a limb of the person being monitored and typically includes various sensors for sensing tampering with the tag and other activities of the monitored person. The sensors periodically transmit corresponding signals, including the tag identification code and the sensor's data. The signals are broadcasted periodically, to be received by one or more local monitoring units, whereby they are processed and optionally reported to the central monitoring station. Current tags typically include microprocessors which enable programming the operation of the tag with regard to various parameters in order to meet the requirements specific to the subject to whom the tag is attached. Thus, parameters such as sampling intervals, data transmission intervals, monitored time periods, permitted and barred locations and a like, can be selected according to the specific needs.
U.S. Pat. Nos. 5,504,474, 5,831,535 and 5,936,529, all assigned to the present assignee and incorporated herein by reference, including drawings and references cited therein, disclose a tamper-resistant tag of the type described above, for use with monitoring systems. U.S. Pat. No. 5,936,529 further teaches a manual resetting device which, when connected to the tag, enables external reprogramming of the tag by authorized parties. Thus, program parameters can be easily reset for reuse with another subject.
Generally, monitoring systems are operated under two basic configurations. One configuration comprises separate independent local monitoring systems, each comprising one or more local monitoring units dedicated to one or more tags which are confined to a pre-determined location. The confined area can vary in size and accordingly the strength of the signal emitted from the tag, or the reception level of the local monitoring unit is set to fit the specific area. The local monitoring units may report to a central monitoring station where information from separate units is collected and compared to the specific program requirements put on the specific monitored person. Alternatively, a local monitoring unit, can serve a separate monitoring station and may process the data independently for further action. Monitoring systems of this type are used for example for criminal offenders under house arrest, where the offender is required to stay in a pre-designated location or for Alzheimer patients where there is a danger of the patient getting lost, and a like. In wake of recent events the system is also used for monitoring patients carrying infectious diseases. Typically, the pre-designated location is the monitored person residence, place of work, confinement area and the like.
Another configuration is an area monitoring system (AMS) where a network of monitoring units “covers” a restricted area in which a plurality of tag carriers are moving. Such systems are employed in building facilities and surrounding, for example hospitals, rehabilitation centers and a like, where a large number of persons are monitored within the same area. The arrangement of the local monitoring units in a monitored area is such that the receiving range of each monitoring unit covers a certain part of the whole area. Preferably the zones covered by adjacent monitoring units overlap such that the signals transmitted by tags are received by more than one monitoring unit. The data collected from all monitoring units is reported to a central monitoring station where, by applying various methods, the location of each of the tag carriers is determined to more or less degree of accuracy.
Where tag carriers are required to stay in pre-designated locations, it is important to determine the location of each tag carrier accurately. Accurate determination of the tag carriers' location is necessary in order to identify whether a tag carrier has out stepped a pre-designated boundary such as a room or other confined space or is approaching a port hole or certain restricted areas. For example: criminal offenders, patients such as mental illness patients or Alzheimer patients or infectious diseases patients, young children and a like may be restricted to a particular room or an area. If such persons exit a room or try to cross a port hole or approach a door the system should be able to alert the restricted actions performed by such monitored persons. In addition, determining an accurate location of a monitored subject is important in cases in which patients need close care and urgent medical treatment upon request.
High accuracy distance estimation methods may employ triangulation calculations based on received signal analysis, time-of-arrival analysis, angle-of-arrival analysis, received signal strength indication (RSSI) or combinations thereof. Simpler “cell based” methods may use a set of rules based on knowledge of the coverage area of each of the monitoring units and of overlapping regions. The simpler “cell based” methods are inherently limited by the number of the local monitoring units distributed within the monitored area and their coverage area. While other known methods suffer from a number of drawbacks. Triangulation methods and angle-of-arrival method may involve special relatively expensive technology such as direction-finding receivers and may suffer from inaccuracy due to screening and reflecting effects, especially when used indoors. The tracking and locating ability of monitoring system, i.e., the ability to continuously locate the position of individuals as they move throughout a restricted zone, depends on the space and time resolution of the information received. Obviously, the higher the number of monitoring units distributed within a confined area and the shorter the time intervals between successive signals, the better is the tracking resolution. However, expense consideration put constraints on the number of monitoring units and power consumption consideration limit the signal transmission repetition rate. The RSSI method uses the fact that the RSSI decreases while the distance between a monitored person and a local monitoring unit increases. An estimation of the distance between a monitored person and a monitoring local unit can be made by using pre measurements of the dependency between the RSSI signal and the distance or by making calculations for finding such dependency. For long distances, the RSSI is weak, relative to the receiver dynamic range and good accuracy distance estimation can be made. However, as a monitored person moves toward a local monitoring unit, the RSSI increases. Decreasing the distance may, further more cause the receiver, which is a part of the local monitoring unit, to saturate. When saturated, the RSSI increases only slightly or may not increase at all, while the distance between the monitored person and the monitoring unit continues to decrease. In fact, two different distances may get the same RSSI values and hence the same distance estimation. To conclude, a saturation effect may cause non accurate distance estimation between a monitored person and a monitoring unit. A combination of an RF and other monitoring system such as an infra red (IR) detector may be used to overcome that problem and to determine the location of the monitored person at short distances. However, IR systems involve special relatively expensive technology and may suffer from inaccuracy due to screening blinding and reflecting effects, both indoors and outdoors. IR systems also require a line of sight which is a significant drawback especially indoors or when the tag is carried in a such a way that its position relative to the fixed reader can not be pre-set (i.e on a person but not necessarily visibly presented at all times). There is therefore a need for a method and apparatus for enhancing the distance estimation accuracy between a monitored person wearing a transmitting tag and a local monitoring unit.
The present invention, by adding a novel feature within the existing basic structure of a local monitoring unit, provides enhancement of tracking and locating capability of a monitoring system. Furthermore, the novel feature of the present invention provides benefits such as, for example, control over accessibility to particular zones or equipment within a restricted area and control over the movements and actions of monitored subjects.